Plasma-assisted etching processes are often used in semiconductor technology for manufacturing LSI circuits in order to produce fine structures. To that end, an etching mask is arranged on a layer to be structured. The structures of the etching mask are to be transferred into the layer to be structured with dimensional accuracy in the anisotropic etching process. A lower layer arranged under the layer to be structured should remain optimally unaffected by the etching process. In addition to the requirement of high anisotropy, a requirement of high selectivity relative to the lower layer is also necessary for the etching process.
This problem is especially serious when the layer to be structured is arranged on an uneven surface. This is usually unavoidable in the manufacture of integrated circuits. In particular, this is the case when structuring a polysilicon layer that is arranged on a thin SiO.sub.2 layer. This example occurs in the manufacture of polysilicon gate electrodes on a gate oxide layer produced surface-wide in the manufacture of logic or memory circuits.
When the surface on which the layer to be structured has vertical side walls, then side wall coverings, referred to as spacers, are formed at these vertical side walls in the anisotropic etching process. The etching process must be continued in order to remove these, even when the lower layer is already uncovered to a great extent. This procedure is referred to as "over-etching". A high selectivity of the etching is required so that the lower layer is thereby not attacked.
It is especially important in the example of a polysilicon layer to be structured on a lower layer composed of SiO.sub.2 for producing a gate electrode to completely remove the side wall coverings, since these would lead to shorts in the finished surface structure.
An article by S. Samukawa et al. (J. Vac. Sci. Technol. B8(6) page 1192, November/December 1990) discloses that the selectivity of an etching process for structuring polysilicon relative to SiO.sub.2 be increased by adding O.sub.2 as an additional process gas to the Cl.sub.2 process gas.
Another possibility is to modify the process chemistry, for example by replacing the chlorine chemistry with a bromine chemistry.
Only a finite increase in the selectivity, however, can be fundamentally achieved with these measures, so that the lower layer must always have a minimum thickness in order to withstand the over-etching during the removal of the side wall coverings. The thickness of the lower layer defines the maximum duration of the over-etching.